The present invention relates to a broach assembly for cutting surfaces in a slot and particularly relates to a broach assembly for use in a turbine for cutting active surfaces of dovetail slots and methods of cutting the slots.
Various machined configurations used for fitting or attachment of fixed or movable parts frequently wear as a function of time and use. For example, mating dovetails are generally employed on the wheels and buckets of a turbine for securing those parts to one another. Conventionally, shapers, planers and milling machines are utilized to form the original dovetails. In a gas turbine, the wheels form separate or discrete parts which are assembled to form the rotor. Once assembled, the original tools utilized to form the dovetail slots cannot be used to repair the slots in assembly, principally due to the proximity of the wheels to one another. Similarly, the dovetail slots on the wheels of steam turbines cannot be readily repaired using such tools for like reasons. Also, the turbines are typically on-site in an electric utility and appropriate machine tools to effect repairs of this type are typically not available at those sites.
Over time and use, it will be appreciated that the contact surfaces between the dovetail slots and the dovetails on the buckets require refurbishing. For example, galling typically occurs where one surface chafes against another, causing spalling and general disruption of the mating surfaces. In the context of both steam and gas turbines, the wheel dovetail slot and bucket dovetail have active (contact) and inactive surfaces. The active surfaces of the dovetail slot and bucket dovetail are those surfaces which engage one another as a result of rotation of the rotor. Inactive surfaces of the dovetail slot and bucket dovetail are those surfaces which are only lightly engaged or spaced from one another while the rotor rotates during active use of the turbine. The active surfaces are typically disrupted over time and use and thus require refurbishment.
In accordance with a preferred embodiment of the present invention, there is provided a multi-part broach assembly for cutting the surfaces of a slot, for example, the active surfaces of a dovetail-shaped slot of a turbine rotor to reestablish these surfaces, contact areas and clearances on axial entry dovetail rotors. The broach assembly is of a size and configuration enabling its use in situ with respect to the turbine wheels. The broach assembly is also self-aligning and can be fabricated inexpensively using conventional technologies. Particularly, the broach assembly comprises a plurality of cutters and spacers each having the general dovetail configuration of the slot in which the active surfaces are to be refurbished. The spacers and cutters are preferably alternately assembled to form a sub-assembly with a lead and a lag also having the general dovetail configuration of the slot through which the broach assembly will be passed to refurbish the active surfaces of the slot. Preferably, a draw bolt is passed through aligned openings of the lead, lag, cutters and spacers. Apertures are also provided on the cutters, spacers, lead and lag for receiving alignment rods on opposite sides of the draw bolt whereby, in assembly, the lead, lag, cutters and spacers are fixed relative to one another. Except along a tapered side of the sub-assembly of cutters and spacers described below, the margins of the spacers are preferably inset from the margins of the adjacent cutters. The margins of the lead are generally smaller than the margins of the following spacer to facilitate lead-in into the slot undergoing repair. The margins of the lag generally correspond to the margins of the preceding cutter and the lag thus precludes lateral skewed movement of the broach assembly upon completing a pass of the assembly through a slot undergoing repair.
The broach assembly is designed for multiple passes through the slot, with each pass or plurality of passes progressively cutting the active surfaces of the dovetail slot. To accomplish this, a side, i.e., a bottom side, of each spacer which faces generally in an opposite direction from the direction in which the active cutting edges on the cutters face, is tapered and projects beyond the bottom side of adjacent cutters. These bottom sides of the spacers are progressively tapered in the sub-assembly of spacers and cutters in a direction from the lead to the lag, the overall height of the assembly increases from lead to lag. More particularly, each tapered surface of each spacer lies along an apex or bottom side of the spacer, i.e., the exposed surface extending along the narrowest portion of the spacer. Tapers are accordingly formed on the apices of the assembled spacers and face in opposite directions relative to the active cutting surfaces of the cutters which cut the active surfaces of the dovetail slots. Thus, as the broach assembly is drawn through the dovetail slot, the cutting edges of the cutters are progressively displaced radially outwardly relative to the bottom of the slot and against the active surfaces of the slot to progressively cut those surfaces.
Preferably, a multiplicity of shims or precision stock of progressively increasing thicknesses are used to progressively displace the broach assembly in a direction outwardly toward the active surfaces of the slot as the broach assembly is drawn through the slot. For example, by placing a shim in the base of the slot and drawing the broach assembly through the slot, the action of the taper on the spacers against the shim displaces the cutting edges of the cutters progressively radially outwardly against the active surfaces of the slot. By replacing the shim with a thicker shim on a subsequent pass or passes of the broach assembly through the slot, additional material is removed from the active surfaces of the slot. Consequently, the active slot surfaces are reestablished, enabling their contact with the active surfaces of the mating part, i.e., the active surfaces of the bucket dovetails.
In a preferred embodiment according to the present invention, there is provided a broach assembly for establishing contact surfaces in a dovetail slot of a turbine wheel with surfaces of a bucket dovetail receivable in the slot, comprising a sub-assembly including a plurality of cutters having cutting surfaces about margins of the cutters and a plurality of spacers interposed between the cutters, a lead and a lag carried by the sub-assembly adjacent opposite ends thereof for respectively leading the sub-assembly into the slot and facilitating withdrawal of the sub-assembly from the slot, each of the lead, lag, cutters and spacers having a dovetail shape generally corresponding to the shape of the dovetail slot, the spacers having an apex, the spacers having progressively tapered surfaces along the apices thereof and in a direction from the lead toward the lag for engaging a surface in the slot enabling generally oppositely facing cutting surfaces of the cutters to progressively cut the contact surfaces of the slot as the assembly passes through the slot lead end first.
In a further preferred embodiment according to the present invention, there is provided a broach assembly for establishing contact surfaces in a slot of a first part with surfaces of a second part receivable in the slot, comprising a plurality of cutters each having an opening therethrough and cutting surfaces about margins of the cutters, a plurality of spacers each having an opening therethrough and interposed between the cutters, an element passing through the openings and cooperable with the cutters and spacers to form a sub-assembly of cutters and spacers, a lead and a lag carried by the sub-assembly adjacent opposite ends thereof for respectively leading the sub-assembly into the slot and facilitating withdrawal of the sub-assembly from the slot, the spacers having progressively tapered surfaces along a side of the sub-assembly and in a direction from the lead toward the lag for engaging a surface in the slot enabling generally oppositely facing cutting surfaces of the cutters to progressively cut the contact surfaces of the slot as the assembly passes through the slot lead end first.
In a further preferred embodiment according to the present invention, there is provided a method of progressively cutting surfaces in a slot of a first part to establish contact with mating surfaces of a second part receivable in the cut slot comprising the steps of providing a sub-assembly of alternating cutters and spacers each having a marginal configuration generally conforming to the configuration of the slot with the spacer margins being inset from the cutter margins, forming tapers along surfaces of the spacers on a side of the sub-assembly facing generally opposite to cutting surfaces formed on the cutters and passing the sub-assembly through the slot with the tapered spacer surfaces engaging a surface in the slot and in a direction enabling the cutting surfaces of the cutters to progressively cut the surfaces of the slot to form contact surfaces for engaging the mating surfaces of the second part upon reception of the second part in the cut slot of the first part.